Formation of catalytic oxide surface on an electrode



3,335,025 FORMATION OF CATALYTIC OXIDE SURFACE ON AN ELECTRODE Robert A. Rightmire, Twinshurg, and Theodore B. Selover, Jr., Cleveland, Ohio, assignors to The Standard Oil Company, Cleveland, Ohio, a corporation of Ohio Filed Mar. 22, 1963, Ser. No. 267,117 3 Claims. (Cl. 117-931) This invention relates to oxide coating, and concerns particularly the formation of oxide coatings on supporting substrate surfaces.

Electrochemical reaction devices using an oxygen electrode require a hard thermally resistant oxide coating, especially a catalytic coating with avariable oxidation state. High melting point refractory oxide coatings are desired.

In certain devices such as electric storage devices and fuel cells, the valence exchange feature of metals with variable oxidation states such as molybdenum, vanadium, tungsten and chromium is employed to enhance the rate or quantum of electron generation.

This application, accordingly, is concerned with the provision of electrodes with coatings of variable oxidation state which may be employed in electrodes for electrochemical reaction processes, particularly for fuel cells, capacitors, electric storage devices, electrochemical reac tion apparatus for selective oxidation-reduction of certain chemical compounds, and other devices where oxygen chemisorption-desorption is desired.

'In carrying out the invention in a preferred form thereof, a suitable medium such as the flame of an oxy-hydrogen or oxyacetylene torch or a plasma arc torch is employed for spraying on to a suitable substrate, in an oxygen rich atmosphere, a substance which forms an effective oxidation catalyst having the properties of the oxides of molybdenum, vanadium, tungsten and chromium. The oxides in accordance with one embodiment are applied directly in the powder form, or the oxides are formed in situ while spraying by fiuidizing the parent metal powder in an oxygen carrier before feeding to the flame or arc, thus oxidizing the metal at the temperature of the coating process. In accordance with another embodiment a suitable compound is decomposed in the vapor phase in an oxygen rich flame or are to form the oxide.

Still another embodiment would include the deposition of an oxide of an inactive metal upon a sub-strate, by the use of a flame or are followed by a post-impregnation of deposited oxide with a solution of a compound of a catalytically active metal.

A better understanding of the invention will be afforded by the following detailed description considered in conjunction with the accompanying drawing constituting a schematic diagram of a plasma arc torch being employed to form a variable oxidation state surface on a substrate.

Coating may be accomplished by the utilization of a plasma-stream torch 13 of the type described in Patent No. 2,922,869 and illustrated therein.

The plasma coating torch 13 comprises a nozzle 14 formed in a copper anode 15 cooperating with a cathode 16 mounted in a copper cathode holder 17. The cathode 16 is composed of thoriated tungsten. The cathode holder 17 is secured in a suitable manner as by means of thread ing it into a non-conducting body 18 to which the copper anode 15 is removably secured. The anode 15 is secured by a body 19 having a lip 21 engaging a shoulder 22 in the copper anode 15.

Suitable means such as bolts (not shown) and a ring gasket (not shown) are provided for securing the bodies United States Patent 18 and 19 together with a water proof seal 23. Aligned passageways 24 and 25 are provided in the bodies 18 and 19 for cooling water. Likewise, a pocket or well 26 for a body of cooling water is formed in the cathode holder 17. A water proof joint between the cathode holder 17 and the body 18 is formed by means of an insulating gasket 27.

The body 18 is so formed as to provide an arc chamber 28 therein surrounding the cathode 16 and a passageway 29 is formed in the body 18 for injecting material to be fed into the chamber 28 for feeding into the jet 39. An inlet tube 31 is connected to the passageway 29 for supplying the arc. operating inert gas, such as argon.

For feeding in certain types of materials which must be fed to the are after it has been formed an alternate injection port 30 may be provided in the anode 15 adjacent the nozzle 14. The injection port 30 is close to the outlet of the nozzle 14.

The anode 15 is connected to a positive current supply terminal 32 by a diagrammatically indicated conductor 33 and the cathode holder 17 is connected to a negative current supply terminal 34 through a diagrammatically indicated conductor 35. Preferably a starter electrode 36 is also provided which is adapted to be connected through a conductor 37 to a source of starting or ignition voltage.

As indicated schematically by the arrow 38, the substrate 11 is adapted to be moved as the coating 12 is applied by the plasma jet or stream 39. As shown, the substrate 11 is mounted 'upon a roller table indicated schematically by rollers 41, driven by suitable means (not shown). 1

The jet-maintaining gas is applied through the tube 31.- If desired, refractory material in the form of powder may be suspended in the gas jet and introduced through the tube 31. Inv cases where the material is of such a nature as to make it most feasible to entrain it in the arc at the nozzle 14, the alternate injection port 30 is employed for entraining the powdered refractory material.

When the substrate 11 is an oxidizable metal or composed of a material capable of reacting with oxygen such as carbon or graphite, the coating is preferably carried out in an inert atmosphere stream or by enclosing the entire assembly in a housing 42. An inert gas such as argon is in this case introduced through an inlet 43. The housing 42 is provided with an outlet 44 for gas from the plasma jet 39 and for any other substances or reaction products. I

The gas supplied in the inlet tube 31 may comprise an inert gas such as argon, helium or nitrogen which, however, is rich in oxygen to prevent thermal dissociation of oxygen atoms from a high oxide to a lower oxide state resulting from the vaporized entrained powder.

' The powder employed is one which will produce an effective oxidation catalyst having catalytic properties similar to tungsten, molybdenum, vanadium or chromium oxides. One of these parent metals may be employed in powdered form and is fluidized in the oxygen carrier brought through the injection port 30 into the oxygen rich arc. The high temperature causes conversion to the oxide which is sprayed by the plasma jet 39 against the surface of the substrate 11 to form the oxide coating 12.

Instead of the parent metal powder, a suitable compound may also be employed which is decomposed by vapor phase decomposition in the oxygen rich flame or are to the desired oxide. Among the compounds which may be used for this purpose are ammonium metatungstate, (NH W O -8H O, ammonium vanadate,

a complex hydrated ammonium molybdate,

6MO7O24 chromium carbonyl, Cr (CO) or many other compounds of similar nature.

The gas flow into the inlet 31 is regulated to permit a current of the order of 300 to 800 amperes to flow with a potential difference between terminals 32 and 34 not exceeding 50 volts, in the case of argon, and not exceeding 175 volts in the case of nitrogen or helium, and to maintain a temperature between 7000 F. and 14,000 F. in the arc 39.

The process of the invention may be illustrated by the following specific example of a process of depositing a vanadium oxide coating on steel.

The cold rolled steel sub-strate surface 11 to be coated was sand blasted, washed with solvent to remove grit and grease, and air dried. This optional pretreatment improves the quality of adhesion of the oxide coating to the steel. The plasma spraying of the oxide powders was carried out with a Giannini Plasmadyne Corporation Model L-40 plasma generating system with an N-4 head. The vanadium oxide (V powder of 150 to+325 mesh particle size was fed to the are 39 by means of a feed hopper (adapted from the Flame Ceramic spray equipment of Continental Coating Corporation) for entraining the finely divided powder in the stream of argon carrier gas supplied through the inlet tube 31.

Four Miller Electric Company rectifiers capable of producing forty-eight kilowatts supplied the electrical power for the are. In this case, the arc gas rate Was 1.0 standard cubic feet per minute of argon, and the powder-carrier gas rate also was 1.0 standard cubic feet per minute of argon at an arc current of 500 amperes and 40 volts. The area of the substrate sheet to be coated was passed rapidly back and forth across the nozzle exit of the plasma are at a distance of three inches from the arc anode while being cooled on the reverse side at the same time by a stream of air.

The coating produced was a V0 blue coating which could be dissolved off in acid to produce the typical vanadyl blue solution. The V0 blue coating could be converted to orange-brown V 0 upon mild heating in. air.

Another specific example, under exactly the same condition with the exception that the carrier gas was air instead of argon, produced an orange-brown coating of V 0 directly. As other examples, ammonium heptamolybdate sprayed in an oxyhydrogen flame gave a molybdenum blue coating when the hydrogen to oxygen ratio was greater that two to one and gave a white coating of molybdenum trioxide when the hydrogen to oxygen ratio was less than two to one.

In another example, the cold rolled steel substrate, in the form of a 1%; inch diameter by inch thick disc, was pretreated with a Pangborn type AY grit blasting machine using No. G-40 angular iron grit. The grit blasted disc was washed with solvent to remove grit and grease and then air dried. The disc was then immediately plasmasprayed with alumina using the Giannini Plasmadyne Corporation Model L-40 plasma generating system with an N-4 head and applying techniques similar to those pre viously described for the plasma spraying of the catalytically active materials. However, in this case, the powder which is sprayed is Davison High Purity Beta Trihydrate Alumina (screened to -140 to +325 mesh) using argon as the carrier gas.

The weight of the alumina coating was determined to be 0.4177 gram. Approximately 0.08 milliliter of a sixty-two percent aqueous solution of chromium nitrate [Cr(NO was distributed evenly over the surface of the alumina deposit by means of a one hundred microliter syringe. A few minutes were allowed for the liquid to become completely absorbed, and then the disc was dried in an electric oven for twenty-five minutes at a temperature of degrees C. This process Was repeated until five approximately equal increments (a total of 0.392 milliliter) of the chromium nitrate solution had been added. After the last incremental addition of chromium nitrate solution had been dried, the disc was placed in an electric furnace and heated for one hour at 1000 degrees C. to decompose the nitrate. This resulted in a coating composition containing 19.3 percent of chromic oxide (Cr O on alumina.

Certain embodiments of the invention and certain methods of operation embraced therein have been shown and particularly described for the purpose of explaining the principle of operation of the invention and showing its application, but it will be obvious to those skilled in the art that many modifications and variations are possible, and it is intended, therefore, to cover all such modifications and variations as fall within the scope of the invention.

What is claimed is:

1. The method of producing electrodes for use in an electrochemical apparatus, said electrodes having variable oxidation state surfaces and produced by the steps of suspending in an oxygen-rich plasma torch are a dry, freeflowing material selected from the group consisting of ammonium metatungstate (NH W O .8H O, ammonium vanadate, NH VO' ammonium molybdate,

and vanadium pentoxide, V 0 and projecting a plasma jet containing suspended decomposition products thereof against a substrate, said surface consisting essentially of said decomposition products.

2. The method of producing electrodes for use in an electrochemical apparatus, said electrodes having variable oxidation state surfaces and produced by the steps of suspending in an oxygen-rich flame of an oxy-hydrogen torch a substance selected from the group consisting of ammonium metatungstate, (NH W O .8H O, ammonium vanadate, NH VO ammonium molybdate,

and vanadium pentoxide, V 0 and projecting the flame and the resulting decomposition product against a sub strate, said surface consisting essentially of said decomposition product. p

3. The method of forming an electrode for use in an electrochemical apparatus said electrode having a variable oxidation state surface and produced by the steps of thermodecomposition of a compound selected from the group consisting of ammonium metatungstate,

ammonium vanadate, NH VO and ammonium molybdate, (NH Mo O .4H O, in a flame spray process and spraying the medium onto the substate, said sufrace consisting essentially of said decomposition product.

References Cited UNITED STATES PATENTS 2,714,563 8/1955 Poorman et al 117160 2,904,449 9/ 1959 Bradstreet 117-46 2,966,575 12/1960 Libby 11793.1 3,183,123 5/1965 Haworth 136-120 OTHER REFERENCES Holland, Vacuum Deposition of Thin Films, published by John Wiley and Sons, page 453 relied on, 1956.

Chemical Abstracts, vol. 55: 11590, 1961.

ALFRED L. LEAVITT, Primary Examiner.

A. GOLIAN, Examiner. 

1. THE METHOD OF PRODUCING ELECTRODES FOR USE IN AN ELECTROCHEMICAL APPARATUS, SAID ELECTRODES HAVING VARIABLE OXIDATION STATE SURFACES AND PRODUCED BY THE STEPS OF SUSPENDING IN AN OXYEN-RICH PLASMA TORCH ARC A DRY, FREEFLOWING MATERIAL SELECTED FROM THE GROUP CONSISTING OF AMMONIUM METATUNGSTATE (NH4)2W4O13$8H2O, AMMONIUM VANADATE, NH4VO3, AMMONIUM MOLYBDATE, 